There are significant challenges associated with designing clot removal devices that can deliver high levels of performance:
There are a number of access challenges that make it difficult to deliver devices. In cases where access involves navigating the aortic arch (such as coronary or cerebral blockages) the configuration of the arch in some patients makes it difficult to position a guide catheter. These difficult arch configurations are classified as either type 2 or type 3 aortic arches with type 3 arches presenting the most difficulty. The tortuosity challenge is even more severe in the arteries approaching the brain. For example it is not unusual at the distal end of the internal carotid artery that the device will have to navigate a vessel segment with a 180° bend, a 90° bend and a 360° bend in quick succession over a few centimeters of vessel. In the case of pulmonary embolisms, access may be gained through the venous system and then through the right atrium and ventricle of the heart. The right ventricular outflow tract and pulmonary arteries are delicate vessels that can easily be damaged by inflexible or high profile devices. For these reasons it is desirable that the clot retrieval device be compatible with as low profile and flexible access and support catheters as possible.
The vasculature in the area in which the clot may be lodged is often fragile and delicate. For example neurovascular vessels are more fragile than similarly sized vessels in other parts of the body and are in a soft tissue bed. Excessive tensile forces applied on these vessels could result in perforations and hemorrhage. Pulmonary vessels are larger than those of the cerebral vasculature, but are also delicate in nature, particularly those more distal vessels.
The clot may comprise any of a range of morphologies and consistencies. Long strands of softer clot material may tend to lodge at bifurcations or trifurcations, resulting in multiple vessels being simultaneously occluded over significant lengths. More mature and organized clot material is likely to be less compressible than softer fresher clot, and under the action of blood pressure it may distend the compliant vessel in which it is lodged. Furthermore the inventors have discovered that the properties of the clot may be significantly changed by the action of the devices interacting with it. In particular compression of blood clot causes dehydration of the clot and results in a dramatic increase in both clot stiffness and coefficient of friction.
The clots may not only range in shape and consistency, but also may vary greatly in length, even in any one given area of the anatomy. For example clots occluding the middle cerebral artery of an ischemic stroke patient may range from just a few millimeters to several centimeters in length.
Stent-like clot retrievers are being increasingly used to remove clot from cerebral vessels of acute stroke patients. These are self expanding devices, similar in appearance to a stent attached to the end of a long shaft, and are advanced through a microcatheter and deployed across clot obstructions in order to trap and retrieve them. They rely on a pinning mechanism to grab the clot by trapping the clot between the self-expanding stent-like body and the vessel wall. This approach has a number of disadvantages:
A stent-like clot retriever relies on its outward radial force (RF) to retain its grip on the clot. If the RF is too low the stent-like clot retriever will lose its grip on the clot, but if the RF is too high the stent-like clot retriever may damage the vessel wall and may require too much force to withdraw. Therefore stent-like clot retrievers that have sufficient radial force to deal with all clot types may cause vessel trauma and serious patient injury, and stent-like clot retrievers that have appropriate radial force to remain atraumatic may not be able to effectively handle all clot types.
The stent-like clot retriever pinning mechanism tends to compress the trapped clot. This compressive force will tend to dehydrate the clot, which in turn tends to increase its coefficient of friction, making it more difficult to remove from the vessel.
Conventional Stent-like clot retriever designs do not retain their expanded shape very well when placed in tension in bends, due to the manner in which their strut elements are connected to one another. This can result in a loss of grip on a clot as the stent-like clot retriever is withdrawn proximally around a bend in a tortuous vessel, with the potential escape of the captured clot. This occurs because the struts of the stent-like clot retriever are placed in tension when it is retracted. This tension is due to friction between the device and the blood vessel, and is increased if an additional load is applied load such as that provided by a clot. In a bend the struts on the outside of the bend are placed in higher tension than those on the inside. In order to attain the lowest possible energy state the outside surface of the stent moves towards the inside surface of the bend, which reduces the tension in the struts, but also reduces the expanded diameter of the stent-like clot retriever.
Another disadvantage with this approach is that it relies on pinning the clot between the stent-like clot retriever and the vessel wall and thus may not restrain the clot effectively when passing a branch vessel or when passing into a vessel that is larger than the fully expanded diameter of the stent-like clot retriever.
Pinning the clot between the stent-like clot retriever and the vessel wall in order to remove it from the vessel also results in high shear forces against the side of the clot as it is removed, potentially releasing fragments of the clot. If these fragments are not retained by the device they may be released leading to further blockages in the distal vasculature.
A particular difficulty encountered when attempting to remove long clots is that conventional devices may be shorter than the clot itself. A device that is shorter than the clot is unlikely to be able to restore flow through the occluded area upon deployment, and thus the pressure gradient across the clot remains a significant impediment to its removal. Simply making such a device longer would likely render it difficult to track through tortuous anatomies and could be traumatic to the vasculature, taking more force to withdraw and potentially getting stuck and requiring surgery to remove.
For many reasons including some or all of the above limitations it is often necessary for a physician to make multiple passes with a clot retrieval device in order to fully remove an obstructive clot. However each time a clot retrieval device is withdrawn the access to the target site is lost. Thus it is necessary to re-advance a guidewire and microcatheter to access and re-cross the clot, and then remove the guidewire and advance the clot retrieval device through the microcatheter. Navigating the guidewire and microcatheter to the clot can take a considerable amount of time especially if the vessels are tortuous. This additional time and device manipulation all adds to the risks to which the patient is exposed.
The challenges described above need to be overcome for any device to provide a high level of success in removing clot, restoring flow and facilitating good patient outcomes. Existing devices do not adequately address these challenges.